Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly ("MEA") consisting of a solid polymer electrolyte or ion exchange membrane disposed between two electrodes formed of porous, electrically conductive sheet material, typically carbon fiber paper. The MEA contains a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane/electrode interface to induce the desired electrochemical reaction. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes to an external load.
At the anode, the fuel permeates the porous electrode material and reacts at the catalyst layer to form cations, which migrate through the membrane to the cathode. At the cathode, the oxygen-containing gas supply reacts at the catalyst layer to form anions. The anions formed at the cathode react with the cations to form a reaction product.
In electrochemical fuel cells employing hydrogen as the fuel and oxygen-containing air (or substantially pure oxygen) as the oxidant, the catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. The ion exchange membrane facilitates the migration of hydrogen ions from the anode to the cathode. In addition to conducting hydrogen ions, the membrane isolates the hydrogen-containing fuel stream from the oxygen-containing oxidant stream. At the cathode, oxygen reacts at the catalyst layer to form anions. The anions formed at the cathode react with the hydrogen ions that have crossed the membrane to form liquid water as the reaction product. The anode and cathode reactions in hydrogen/oxygen fuel cells are shown in the following equations: EQU Anode reaction: H.sub.2 .fwdarw.2H.sup.+ +2e.sup.- EQU Cathode reaction: 1/2O.sub.2 +2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 O
In typical fuel cells, the MEA is disposed between two electrically conductive plates, each of which has at least one flow passage engraved or milled therein. These fluid flow field plates are typically formed of graphite. The flow passages direct the fuel and oxidant to the respective electrodes, namely, the anode on the fuel side and the cathode on the oxidant side. In a single cell arrangement, fluid flow field plates are provided on each of the anode and cathode sides. The plates act as current collectors, provide support for the electrodes, provide access channels for the fuel and oxidant to the respective anode and cathode surfaces, and provide channels for the removal of water formed during operation of the cell.
Two or more fuel cells can be connected together, generally in series but sometimes in parallel, to increase the overall power output of the assembly. In series arrangements, one side of a given plate serves as an anode plate for one cell and the other side of the plate can serve as the cathode plate for the adjacent cell. Such a series connected multiple fuel cell arrangement is referred to as a fuel cell stack, and is usually held together by tie rods and end plates. The stack typically includes manifolds and inlet ports for directing the fuel (substantially pure hydrogen, methanol reformate or natural gas reformate) and the oxidant (substantially pure oxygen or oxygen-containing air) to the anode and cathode flow field channels. The stack also usually includes a manifold and inlet port for directing the coolant fluid, typically water, to interior channels within the stack to absorb heat generated by the exothermic reaction of hydrogen and oxygen within the fuel cells. The stack also generally includes exhaust manifolds and outlet ports for expelling the unreacted fuel and oxidant gases, each carrying entrained water, as well as an exhaust manifold and outlet port for the coolant water exiting the stack. It is generally convenient to locate all of the inlet and outlet ports at the same end of the stack.
Perfluorosulfonic ion exchange membranes, such as those sold by DuPont under its Nafion trade designation, must be hydrated or saturated with water molecules for ion transport to occur. It is generally accepted that such perfluorosulfonic membranes transport cations using a "water pumping" phenomenon. Water pumping involves the transport of cations in conjunction with water molecules, resulting in a net flow of water from the anode side of the membrane to the cathode side. Thus, membranes exhibiting the water pumping phenomenon can dry out on the anode side if water transported along with hydrogen ions (protons) is not replenished. Such replenishment is typically provided by humidifying the hydrogen-containing fuel stream prior to introducing the stream into the cell. Similarly, the oxygen-containing oxidant stream is typically humidified prior to introducing the oxidant stream into the fuel cell to prevent the membrane from drying out on the cathode side.
In U.S. Pat. No. 5,260,143, issued Nov. 9, 1993, it was disclosed that a new type of experimental perfluorosulfonic ion exchange membrane, sold by Dow under the trade designation XUS 13204.10, did not appear to significantly exhibit the water pumping phenomenon in connection with the transport of hydrogen ions across the membrane. Thus, the transport of water molecules across the Dow experimental membranes did not appear to be necessary for the transport of hydrogen ions as in the Nafion-type membranes. Nevertheless, the transport of hydrogen ions across the Dow membrane requires the membrane to be at least partially saturated to the extent that hydrogen ion transport across the Dow membrane will cease if the membrane dries out.
Thus, hydrogen ion conductivity through ion exchange membranes generally requires the presence of water molecules between the surfaces of the membrane. The fuel and oxidant gases are therefore humidified prior to introducing them to the fuel cell to maintain the saturation of the membranes within the MEAs. Ordinarily, the fuel and oxidant gases are humidified by flowing each gas on one side of a water vapor exchange membrane and by flowing deionized water on the opposite side of the membrane. Deionized water is preferred to prevent membrane contamination by undesired ions. In such membrane-based humidification arrangements, water is transferred across the membrane to the fuel and oxidant gases. Nafion is a suitable and convenient humidification membrane material in such applications, but other commercially available water exchange membranes are suitable as well. Other non-membrane based humidification techniques could also be employed, such as exposing the gases directly to water in an evaporation chamber to permit the gas to absorb evaporated water.
It is generally preferred to humidify the fuel and oxidant gases at, or as close as possible to, the operating temperature and pressure of the fuel cell. The ability of gases such as air to absorb water vapor varies significantly with changes in temperature, especially at low operating pressures. Humidification of the air (oxidant) stream at a temperature significantly below fuel cell operating temperature could result in a humidity level sufficiently low to dehydrate the membrane when the stream is introduced to the cell. Consequently, it is preferable to integrate the humidification function with the active section of the fuel cell stack, and to condition the fuel and oxidant streams to nearly the same temperature and pressure as the active section of the stack. In such an integrated arrangement, the coolant water stream from the active section, which is at or near the cell operating temperature, is normally used as the humidification water stream.
In conventional fuel cell stack designs, such as, for example, the fuel cell stack described and illustrated in U.S. Pat. No. 5,176,966, issued Jan. 5, 1993, the fuel and oxidant streams are typically directed via manifolds or headers through the active section, without participating in the electrochemical reaction, to condition each stream to cell temperature prior to introducing them to the humidification section. In such conventional stack arrangements, the humidification section is located downstream from the active section, so that the reactant streams can first be heated to approximately the cell operating temperature in the manifolds passing through the active section. Once heated to cell operating temperature, the reactant streams can absorb water vapor in the humidification section so that when the humidified fuel and oxidant streams are returned to the active section and fed to the respective anode and cathodes, dehydration of the membrane is avoided.
While location of the humidification section downstream from the active section has the advantage of conditioning the reactant streams to cell temperature prior to humidifying the reactant streams in the humidification section, a downstream humidification section configuration has a significant disadvantage, namely, the manifold passing through the active section for introducing the reactant streams to the humidification section occupies space within each plate forming the active section of the stack. The use of that space by the manifolds carrying the reactant streams through the active section to the humidification section reduces the amount of area on each plate otherwise available to participate in the electrochemical reaction.
A conventional fuel cell stack with a humidification section located downstream from the active section requires a total of nine manifold openings in each plate forming the active section: one manifold opening for each of the inlet fuel, inlet oxidant and inlet coolant water streams; one manifold opening for each of the humidified fuel and humidified oxidant streams and for the outlet coolant/inlet humidification water stream; and one manifold opening for each of the outlet fuel, outlet oxidant and outlet humidification fluid streams. These nine manifold openings are generally located and arranged in the corners of each of the plates forming the active section.
In the present invention, the humidification section is located upstream from the active section, thereby reducing the number of manifold openings in the plates forming the active section from nine to six. It has been found that the inlet fuel and inlet oxidant streams either do not require conditioning in the active section or can be conditioned (heated) outside the stack before introducing them to the humidification section of the stack. The six manifold openings in the active section of the present invention are the humidified fuel, the humidified oxidant and the inlet coolant streams, and the outlet fuel, outlet oxidant and outlet coolant/inlet humidification fluid streams. The reduction in the number of manifold openings from nine in conventional stacks to six in the present invention increases the amount of area available on the active section plates to participate in the electrochemical reaction. While the number of manifold openings in the active section plates is reduced from nine to six in the present invention, the humidification section has nine manifold openings which correspond to the nine manifold openings in the active section plates of conventional fuel cell stacks.
Accordingly, it is an object of the present invention to reduce the number of manifold openings in the plates forming the active section of the fuel cell stack by locating the humidification section upstream from the active section.
It is also an object of the invention to increase the amount of electrochemically active area on each of the plates forming the active section of the fuel cell stack.